Programmable and convertible non-volatile memory array

ABSTRACT

A non-volatile, integrated circuit memory, such as a Flash EPROM, including an array 1 of memory cells 10, each cell having a floating gate 14 for programming the cell and a control gate 11 for reading the cell, the array having a plurality of row lines 15, a plurality of column lines 25 and a plurality of output lines 18. Included is a decoder circuit 16 having a plurality of input lines 94, 96, for each row in the array, and having as outputs the row lines 15. The decoder circuit includes a decoder logic circuit associated with each row line, the decoder logic circuit including a plurality of low power logic devices 84-90 interconnected to perform a predetermined decoding function on the signals on the input lines for the associated row line to apply a signal to an associated row node when the decoder logic circuit determines that the associated row line is selected. The decoder circuit also includes a high power pass device 82 associated with each row line, having one of its source and drain connected to the associated row node, having the other of its source and drain connected to a row line and having its gate connected to a first voltage, lower than the high voltage, so as to couple the signal on the row node to the row line. Finally, a keeper circuit 102 is provided, associated with each row line and coupled to the high voltage for sensing the signal on the row line and in response thereto for coupling the high voltage to the row line. The memory has high speed, since the decoder logic is performed by low power devices. In addition, in fabrication the memory is readily convertible to a permanent ROM by eliminating the formation of the floating gate, bypassing the high power pass device and not connecting the keeper circuit.

This application claims the benefit of U.S. provisional application No.60/001,236, filed Jul. 19, 1995, by Smayling et al.

TECHNICAL FIELD OF THE INVENTION

This invention relates to integrated circuit processes and integratedcircuits produced thereby, and more particularly to non-volatile memorycells, such as EPROMs, EEPROMs and Flash EPROMS, and the fabrication ofarrays thereof.

BACKGROUND OF THE INVENTION

Several types of programmable, non-volatile memory cells are known andused today. The Erasable Programmable Read Only Memory, or EPROM, cellsare memory cells including a field effect transistor, or FET device,that is provided with a floating gate that can be charged, on anon-volatile basis, with electrons using avalanche injection. A devicehaving such a charged floating gate will not conduct, even when itscontrol gate has a positive, read-level voltage applied thereto, whileone not having such a charged floating gate will conduct when suchvoltage is applied to its control gate. Thus, an array of such cells canbe selectively programmed.

The charge remains on the floating gate indefinitely, for all practicalpurposes, unless the array is erased. Such arrays are erased by theapplication of ultraviolet radiation thereto. The Electrically ErasableProgrammable Read Only Memory, or EEPROM, cells are memory cells thatare similar to EPROMs in that they include a field effect transistor, orFET device, that is provided with a floating gate that can be chargedwith electrons using avalanche injection, but each cell is individuallyerasable electrically. EEPROM arrays are thus more flexible than EPROMarrays, but are significantly larger than EPROM arrays. Finally, FlashEPROMS, sometimes referred to as Flash memories, also have a floatinggate that can be charged using avalanche injection. However, Flash EPROMcells are erasable using Fowler-Nordheim tunnelling, an electricaleffect rendering unnecessary the application of light to the array. InFlash EPROM arrays, as in EPROM arrays, the entire array is erased,rather than individual cells as with EEPROMs. All of the aforementionedarrays are referred to collectively herein as Non-Volatile ProgrammableArrays, or NVPAs.

A disadvantage of all of these NVPA arrays is that rather high voltagesmust be applied to the cells to program them and, in the case of EEPROMsand Flash memories, to erase them. These voltages are typically in theorder of fifteen volts. As a consequence, the devices involved inderiving the row and column signals have to be made quite large towithstand these voltages. Device lengths of the order of two microns aretypical for such high voltage devices. Such devices not only take upmore space than low voltage devices, which, by contrast have channellengths of the order of 0.8 microns, but they also have significantlyslower access times than low voltage devices. Moreover, semiconductorprocessing technology advances are driving device sizes smaller andsmaller. While this is providing benefits such as including fasterdevices as well as allowing more devices on a chip of a given size, thehigh voltage devices do not, as a rule, scale with the low voltagedevices. This is because the programming and erasing voltages requiredare still of the same order in the smaller technologies. Thus, the highvoltage devices must remain roughly the same size even in these smallertechnologies. As a result, as device dimensions shrink with technologyadvances, NVPA arrays cannot be shrunk to the degree desired, and speedimprovements are less than desired, as well.

Another problem in the use of NVPA arrays arises from their application.For example, embedded Flash memory and EPROM memories are commonly usedin digital signal processors, or DSPs, and in microcontroller units, orMCUs, for prototyping by the developers of these complex chips. Forapplications in which the embedded array stores only programinformation, and no data, a permanent ROM, or Mask ROM, version of thechip is later developed to give lower cost in high production volume.However, this requires replacing the NVPA module with a completelydifferent ROM array module. This, in turn, requires new chip routing,floorplanning and the like, which is time consuming, expensive, andoften results in using a ROM module which is larger than the NVPAmodule, since the typical ROM, having Contact or Via programming, has alarger cell size than that of an NVPA.

A further problem, in particular with respect to Flash memory, is that,while it was mentioned above that erasure of these memories is typicallyof the entire array, it is desirable in many applications to be able toerase at least segments of the array only, so as to preserve theprogramming in non-selected segments. Segmenting of Flash memories isknown. However, this requires dividing the array into separate physicalareas for each logical sector, which adds delay to the Read path andalso causes a penalty in silicon area.

Thus, there is a need for improvements to Non-Volatile ProgrammableArrays of memory cells to overcome the aforementioned problems. Thepresent invention accomplishes this.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method forconverting an integrated circuit including a non-volatile programmablememory containing predetermined programming to a read-only memory (ROM)programmed according to the predetermined programming, comprising thefollowing steps. First, a first integrated circuit is

fabricated including a memory having the following elements. The memoryincludes an array of memory cells, each cell having a floating gate forprogramming the cell and a control gate for reading the cell, the arrayhaving a plurality of row lines, a plurality of column lines and aplurality of output lines. Included is a decoder circuit having aplurality of input lines to select each row in the array, and having asoutputs the row lines. The decoder circuit includes a decoder logiccircuit associated with each row line, the decoder logic circuitincluding a plurality of low voltage logic devices interconnected toperform a predetermined decoding function on the signals on the inputlines for the associated row line to apply a signal to an associated rownode when the decoder logic circuit determines that the associated rowline is selected. The decoder circuit also includes a high voltage passdevice associated with each row line, having one of its source and drainconnected to the associated row node, having the other of its source anddrain connected to a row line and having its gate connected to a firstvoltage, lower than the high voltage, so as to couple the signal on therow node to the row line. Finally, a keeper circuit is provided,associated with each row line and coupled to the high voltage forsensing the signal on the row line and in response thereto for couplingthe high voltage to the row line. The predetermined programming isverified using the foregoing memory. Second, a second integrated circuitis fabricated, like said first integrated circuit except that the gateof the high voltage pass device may be left unconnected to other wiring.Then, the source and drain of said high voltage pass device are shortedtogether so as to eliminate the function of said high voltage passdevice, and the row line is left unconnected to said keeper circuit.Finally, the memory cells are formed without a floating gate.

These and other features of the invention that will be apparent to thoseskilled in the art from the following detailed description of theinvention, taken together with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a representation of an array of memory cells and associatedcircuitry according to this invention;

FIG. 2 is a plan view of a part of a semiconductor chip having memorycells according to one embodiment;

FIGS. 3(a)-3(k) are elevation views in section of the semiconductordevice of FIG. 2, taken along the line 1--1 of FIG. 2 at various stagesof construction;

FIG. 4 is a schematic diagram of a prior art decoder circuit;

FIG. 5 is a schematic diagram of the decoder circuit of the preferredembodiment of the present invention in which Flash memory cells are usedin Array 1;

FIG. 6 is a circuit diagram of a small portion of the array of thepreferred embodiment of the present invention, Array 1, in which Flashmemory cells are used;

FIG. 7 is a circuit diagram of a small portion of Array 1, in whichpermanent ROM cells are used;

FIG. 8 is a circuit diagram like that of FIG. 5, but in which permanentROM cells are used in Array 1;

FIG. 9 is a plan view of a small portion of Array 1 in an intermediatestage of processing;

FIG. 10 is a plan view of the portion of Array 1 shown in FIG. 9, at alater stage of processing than that of FIG. 9; and

FIG. 11 is a circuit diagram of a small portion of Array 1, showing arepresentation of the back gates of the individual cells, and theirinterconnectedness.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Process Overview

The process used in the manuufacture of the preferred embodimentincludes forming a floating-gate cell, a line of such cells, or an arrayof such cells, in an isolated well. At the same time, high-voltage andlow-voltage logic transistors are formed. During an erasing operationthe source of each memory cell to be erased is driven to a firstpositive voltage while the control gate is at reference voltage. Usingthe inventive isolated-well disclosed herein, the drain and the channelof each cell is also driven to a voltage nearly equal to the firstpositive voltage by driving the isolated well to a second positivevoltage that is equal to the first positive voltage, thus eliminatingthe field-plate breakdown-voltage problem. Because there is no need fora diffused source-junction erase window under the floating gate, eachfloating-gate cell is a one-transistor cell having roughly the same areaas that of an ultra-violet-erasable EPROM cell made using the sametechnology. Without the prior-art requirement for a separate tunnellingregion near the source, a masking step and a phosphorus implant areeliminated. The preferred embodiment is realized in an X-cell memoryarray that has the small size of an ultra-violet-erasable EPROM and thathas manufacturing complexity slightly greater than that of anultra-violet-erasable EPROM. The high-voltage N and P-channeltransistors and low voltage N and P-channel transistors of amicrocontroller are formed on the chip as the memory cells are formed.

The nonvolatile memory array is encased in a P-well, and the P-wellencased in a deep N-well, the two wells separating the memory array fromthe integrated circuit substrate and from the other circuitry of theintegrated circuit. At the same time the deep N-well is formed for thenonvolatile memory array, deep N-wells are formed for the high-voltageP-channel transistors of the logic circuitry. At the same time theP-well is formed for the nonvolatile memory array, P-wells are formedfor the low-voltage N-channel transistors.

With the control gate and the integrated circuit substrate at 0V, thedeep N-well allows application of a positive voltage of perhaps +16V tothe source/drain a diffusions and the P-well of the nonvolatile memoryarray during erasure. Alternatively, with the substrate at 0V, a smallerpositive erasure voltage (perhaps +10V) is applied to the source/draindiffusions and the P-well, and a negative erasure voltage (perhaps -6V)is applied to the control gate. Application of those voltages permitsthe cells of the memory array to be erased without the causingfield-plate stress at the p-n junctions between the source/draindiffusions and the P-well.

The tern "well" as used herein refers to a relatively large diffusionregion formed in a semiconductor substrate. Such diffusion regions aresometimes referred to as "wells", "tanks" or "tubs". The "wells","tanks" or "tubs" are generally large enough to contain the diffusionregions and channels of active circuit elements.

The process results in a memory array with rows and columns of cellshaving a size and structure similar to those of a prior-artultra-violet-erasable X-type arrays and includes high- and low-voltagelogic circuitry on the same chip. The final device combines logictransistors and a memory with a dense flash EPROM circuitry, both formedwith the manufacturing ease of that for an ultra-violet-erasable EPROMstructure.

Process Details

Referring now to FIG. 1, a memory device is shown which has an Array 1of rows and columns of memory cells 10, each of which is an insulatedgate field effect transistor having a control gate 11, a source 12 and adrain 13. The cells 10 include a floating gate 14 between the controlgate 11 and the channel between source 12 and drain 13.

The control gates 11 of all cells in each row are connected to one of aset of row lines 15. Row lines 15 are connected to an X address decoder16 which selects one of row lines 15 based on a row address on lines 17.In a read operation, the selected one of the lines 15 goes high, theothers remain low.

The drains 13 of adjacent cells 10 are connected in common to Y outputlines 18. The lines 18 are connected through Y output select transistors19 to a Y output line 20. The gates of the transistors 19 are connectedto a Y address decoder 21 via lines 22 which function to apply asupply-level voltage V_(DD) (about +3 to +5 volts) to one of the lines22 and hold the others at ground based on an address input on lines 23.

The sources 12 of adjacent cells 10 are connected in common to anotherset of column lines 25 which function as virtual ground lines. Each line25 is connected through a column select transistor 27 to ground. Thegates of all of these transistors 27 are connected via lines 28 to aground selector 29 which receives the output lines 22 from the Y addressdecoder 21, along with the least significant address bit A_(o) and itscomplement A_(o--), and functions to activate only one of the lines 28for a given Y address.

In the read mode, the X address decoder 16 functions, in response to rowline address signals on lines 17 and to a signal from a microprocessor,to apply V_(DD) to the selected row line Xa (and, thus, the selectedcontrol gate 11), and to apply ground to deselected row lines 15. Rowline Xa is one of row address lines 15. The Y address decoder 21functions, in response to column address signals on lines 23, to turntransistor 19a on by applying V_(DD) on line 22a, causing a senseamplifier (not shown) connected to the DATA OUT terminal to apply apreselected positive voltage Vsen (about +1 to +1.5 volts) to theselected drain-column line 18a. Deselected drain-column lines 18 may beallowed to float (connected to the high impedance of off transistors19), disconnected from the sense amplifier. The ground select circuit 29functions to turn transistor 27a on, connecting the particularsource-column line 25a to ground. All other source-column lines 25 areconnected to Vx, which is at the same level as V_(SEN), through thisassociated transistors 26. At the same time, line 7a operates to turntransistors 26a off. All other transistors 26 are on at this time,causing all deselected source-column lines 25 to be at Vx. Theconductive or nonconductive state of the cell 10a connected to theselected drain-column line 18a and the selected row line Xa is detectedby the sense amplifier connected to the DATA OUT terminal.

In a write or program mode, the X address decoder 16 may function, inresponse to row line address signals on lines 17, and to signals from amicroprocessor, to place a preselected first programming voltage V_(GG)(about +11 to +13V) on a selected row line Xa, including thecontrol-gate conductor 11 of selected cell 10a. Y address decoder 21also functions to place a second programming voltage Vpp (about +5 to+8V) on a selected drain-column line 18a and, therefore, the drainregion 13 of selected cell 10a. Deselected drain-column lines 18 arefloated. The selected source-column line 25 is connected to groundthrough transistor 27a. Deselected source-column lines 25 are allowed tofloat. Deselected row lines are grounded. These programming voltagescreate a high current (drain 13 to source 12) condition in the channelof the selected memory cell 10a, resulting in the generation near thesource-channel junction of channel-hot electrons and/oravalanche-breakdown electrons (hot carriers) that are injected acrossthe channel oxide to the floating gate 14 of the selected cell 10a. Theprogramming time is selected to be sufficiently long to program thefloating gate 14 with a negative potential of about -2V to -6V withrespect to the channel region. The electrons injected into the floatinggate 14, in turn, render the source-drain path under the floating gate14 of the selected cell 10a nonconductive, a state which is read as a"zero" bit. Unprogrammed cells 10 have source-drain paths under thefloating gates 14 that remain conductive, and those cells 10 are read as"one" bits.

During the program and read operation examples described above, cells 10located in P-wells 33 and N-wells 31 (see FIG. 2) are programmed anderased with the P-wells 33 and N-wells 31 at 0V.

Placing Array 1 in the low voltage P-well 33, with the high voltageN-well 31 encasing P-well 33, all in a P-type substrate 30, allows anovel electrical erase method for the Array 1, when Array 1 is comprisedof Flash memory cells. In this mode, P-well 33 and N-well 31 are shortedtogether, and a special erase voltage V_(EE) of about +16V is used. TheP-well 33 is electrically, the "Back Gate" of the cell 10. By biasingthe Back Gate (P-well 33) to V_(EE) and the control gate 11 to zerovolts, the cell can be erased using Fowler-Nordheim tunneling.Heretofore, applying such voltage levels to the cell's well would haverequired applying the voltage to the substrate, which is not feasible.

The set of voltages involved in this mode of erasure are as shown in thefourth column of Table 1.

This mode of erasure further allows sectoring for selective erasurewithout requiring physical separation of the sectors, while allowingselection using zero volts or V_(EE) on the word lines. The sectoring isby row. The selected row has its row line 15 at zero volts, with thenon-selected row lines 15 being at V_(EE) to protect their floating gatecharge.

The terms "source" and "drain", as used herein, are interchangeable. Forexample, the voltages applied to the source 12 regions and the drain 13regions of the memory cells 10 my be interchanged in the read exampleabove.

For convenience, a table of read and write voltages is given in theTable 1 below:

                  TABLE 1                                                         ______________________________________                                        Connection     Read      Write   Flash Erase                                  ______________________________________                                        Selected Row Line                                                                            3-5 V     11-13 V 0 V (All)                                    Deselected Row Lines                                                                         0 V       0 V     +16 V                                        Selected Source Line                                                                         0 V       0 V     Float or +16 V                                                                (All)                                        Deselected Source Lines                                                                      Vx        Float   N/A                                          Selected Drain Line                                                                            1-1.5 V 5-8 V   Float or +16 V                               Deselected Drain Lines                                                                       Float     Float   N/A                                          P-well         0 V       0 V     +16 V                                        N-well         0 V       0 V     +16 V                                        ______________________________________                                    

A method of making the devices of FIG. 1 will be described in referenceto FIGS. 2 and 3a-3k. FIGS. 3a-3k are elevation views in section of thesemiconductor device of FIG. 2, taken along the line 1--1 of FIG. 2 atvarious stages of construction. The method description relates only tothe process for forming an X-cell array of cells 10 and for forming boththe high-voltage P-channel transistors HVPT and low-voltage N-channeltransistors LVNT of the logic circuitry on the same chip. While logiccircuitry normally includes high-voltage N-channel transistors HVNT andlow-voltage P-channel transistors LVPT, the additional steps used tofore such high-voltage N-channel transistors HVNT and low-voltageP-channel transistors LVPT are well known and are therefore not includedin the following discussion.

The starting material is p-epi on a wafer of p+substrate 30, only a verysmall portion shown in the figures. The wafer is perhaps 8 inches indiameter, while the portion shown in FIG. 2 is very small fraction ofthat wafer. A pad oxide PO of about 400 Angstrom (A) is grown on thesurface.

Referring now to FIGS. 3a, and 3b deep N-wells 31 are formed in thesubstrate 30 using the following process. Deep N-wells 31 are patternedwith photoresist PR. The length and width of the implant area in theregion where the memory cells 10 are to be formed must be sufficientlylarge that the dimensions encase a P-well 33 which in turn encases thememory array 1 or a subarray such as, e.g., a row. The length and thewidth of each implant area in the region where a high-voltage P-channeltransistor HVPT is to be formed must be sufficiently large that thedimensions encase the source 12 and drain 13 of each of that transistorHVPT. The N-well 31 implant is then conducted, preferably withphosphorus P at a dose of about 4.0×10¹² ions/cm² and at an energy levelof about 80 KeV. The photoresist is then stripped. An anneal of theN-well 31 dopant is performed at high temperature, perhaps 1200° C. for700 minutes in a nitrogen atmosphere, to form a junction perhaps 7microns (μm) deep. This completes creation of deep N-well regions 31.The implantation defines the channel regions of high-voltage P-channeltransistors HVPT.

Referring now to FIG. 3c and 3d, P-wells 33 are formed in each N-well 31where the memory is to be formed and in each region where a low-voltageN-channel transistor LVNT is to be formed. The P-wells 33 are patternedwith a photoresist layer PR and a P-type implant is performed,preferably with boron B at a dose of about 6.0×10¹² ions/cm² and anenergy of approximately 40 KeV. In regions where the memory array 1 isto be formed, the length and width of the pattern must be sufficientlysmall to allow the P-well 33 to be encased by the deep N-well 31, butsufficiently large to encase the memory array (or sub-array). The depthof P-well 33 must not exceed the depth of N-well 31. The length and thewidth of each implant area in the region where a low-voltage N-channeltransistor LVNT is to be formed must be sufficiently large that thedimensions encase the source 12 and drain 13 of each transistor LVNT.The implantation, defines the channel Ch regions of the memory cells 10and of low voltage transistors LVNT. The photoresist layer is thenstripped. An anneal of the P-well 33 dopant is performed at hightemperature, perhaps 1100° C. for about 500 minutes in a nitrogenatmosphere, to form a junction perhaps 2 μm deep.

Referring to FIGS. 3e and 3f, further processing is described. Aconventional nitride/oxide masking layer NOM is deposited (FIG. 3e) andpatterned to define oxide regions 41 (FIG. 3f). Oxide regions 41 aregrown by localized oxidation (LOCOS) to a thickness in the range ofabout 6300 to 7800 A (the thicknesses of the sections shown in FIGS.3e-3k not being to scale). The growth occurs under an oxidizingatmosphere such as steam for about 600 minutes at about 900° C. Thethermal oxide grows beneath the edges of the mask, creating a "bird'sbeak" instead of a sharp transition. The masking layers are removedusing a hydrofluoric acid dip followed by a hot phosphoric acid etch.

After a cleanup step, a pre-gate oxide layer (not shown) is grown on theexposed silicon surface to a thickness of about 300 A.

At this point, a threshold-voltage-adjust implant may be performed inactive areas including where channels Ch of memory cells 10 are to belocated, those areas patterned using photoresist. For example, boron maybe implanted in the memory cell regions at a dose of about 1×10¹²ions/cm² and at an energy level of about 40 KeV. The photoresist isstripped and the oxide over the active areas is stripped.

Referring to FIG. 3g, oxide is regrown over the structure usingconventional techniques to form a relatively thin gate insulator layer43 approximately 105 A thick. A first polycrystalline silicon layer("poly 1") 14 about 1500 A thick, which will become floating gates ofmemory cells 10 is deposited over the face and is doped to be N+ usingphosphorus. The first polysilicon layer 14 is patterned with aphotoresist and strips are etched to partially form what will befloating gates of the memory cells 10. At the same time, the firstpolysilicon layer 14 is removed form the region where logic transistorssuch as high-voltage P-channel transistors HVPT and low-voltageN-channel transistors LVNT are to be formed. This step is followed by aphotoresist strip and clean-up.

Referring again to FIG. 3g, inter-level insulator layer 45 is thenformed over the structure in the areas where memory cells 10 are to beformed. Inter-level insulator layer 45 may be formed by growing an oxidelayer to about 120 Å, then depositing a nitride layer about 150 Å thick.The equivalent oxide thickness of the inter-level insulator is about 200Å. The poly1 and inter-level insulator are etched. The patternedphotoresist for this step is stripped.

Referring again to FIG. 3g, a second polycrystalline silicon layer("poly 2") 15 about 4500 A thick, which will become control gates/rowlines of the memory array 1 and the gates of high-voltage P-channeltransistors HVPT and low-voltage N-channel transistors LVNT of the logiccircuitry, is then deposited over the face of the wafer and is highlydoped with phosphorus to be N+.

Referring now to FIG. 3h, after de-glazing and patterning withphotoresist, the gates of high-voltage P-channel transistors HVPT andlow-voltage N-channel transistors LVNT of the logic circuitry are etchedin the logic area of the chip. After again patterning with photo resist,a stack etch of (i) the second polysilicon layer 11,15, (ii) theinter-level insulator layer 45, and (iii) the first polysilicon strips14 is performed in the memory area of the chip. This stack etch definesa plurality of elongated control gates 11/row lines 15. The row lines 15connect rows of memory cells 10. This same stack etch separates anddefines the remaining edges of the floating gates 14.

Referring again to FIG. 3h, a photoresist layer PR is deposited andpatterned to open a window over the entire flash array 1. An arsenicimplant As is performed at a dosage of about 5×10¹⁵ ions/cm² at 120 KeVat zero degrees to the normal to create the sources 12 and drains 13 ofmemory cells 10.

Oxide is deposited and removed in conventional manner to form sidewallspacers 50 A cap oxide (not shown) about 300 Å thick is deposited overthe surface.

Referring to FIG. 3i, an arsenic implant As is performed at a dosage ofabout 3×10¹⁵ ions/cm² at 120 KeV, using photoresist PR to protect areasof the chip not implanted, to create the sources 12 and drains 13 of thelow-voltage N-channel transistors LVNT and the N+ region 52 is used tocontact the N- well 31.

Referring to FIG. 3j, a boron implant B is performed at a dosage ofabout 4×10¹⁴ ions/cm² at 20 KeV, using photoresist PR to protect areasof the chip not implanted, to create the sources 12 and drains 13 of thehigh-voltage P-channel transistors HVPT, and the P+ region 54 is used tocontact the P-well 33.

Referring to FIG. 3k, the dopants of memory cells 10, of low-voltage isN-channel transistors LVNT and high-voltage P-channel transistors HVPTare driven with an anneal step at perhaps 900° C. for 20 minutes tocomplete formation of sources 12 and drains 13. A borophosphosilicateglass (BPSG) layer (not shown) may then be deposited over the face ofthe slice. Following the BPSG deposition, the substrate 30 is heatedagain at about 900° C. for about one hour in an annealing ambient toprovide BPSG densification, repair implant damage and junction profiledrive. Column lines 18 and 25 are formed from a layer or aluminum afteretching holes to sources 12 and drains 13 and other place on the chipwhere connection is desired. At the same time that column lines 18 and25 are formed, other conductors are formed for logic circuitry.Off-array contacts for both memory and logic are masked and etchedthrough the BPSG layer.

One problem with an isolated P-well 83 is high well resistance. The highwell resistance causes a significant voltage drop during programming.The voltage drop is decreased by the P+ contact areas 54, which shouldbe strips, preferably extending along at least one side of each P-well33.

Metal is deposited, masked and etched to form metal lines to respectivediffused regions, such as terminals 52 and 54 and to the sources 12 anddrains 13. Additional layers of dielectric and metal are deposited,patterned and etched as required for interconnect. This is followed by aprotective overcoat process.

Decoder

The preferred embodiment includes a novel decoder circuit that isolatesall decoding logic from the high voltages involved in programmingNon-Volatile Programmable Arrays, allowing faster decoding as well asdecoding speed that scales with process improvements, and also allowingeasy conversion of a NVPA to a ROM array, as will be made clear by thedescription below.

Referring now to FIG. 4, there is shown an exemplary prior art X AddressDecoder, made up of high voltage N-Channel devices 60, 62 and 68, andhigh voltage P-Channel devices 64 and 66, all interconnected as shown.In addition, an input 70 is provided to receive a first logic signal, aninput 72 is provided to receive a second logic signal and an input 74 isprovided to receive a third logic signal. The first logic signal is theinverse of the second logic signal. Supply-level voltage Vdd is appliedto input 76. High voltage V_(GG) is provided to port 78, port 80 beingconnected to the ground line. The output 15 is a row line (see FIG. 1).

The voltages applied during programming and during Read to the inputs,and the resultant voltage on the row line 15, are all shown in thefollowing Table 2:

                  TABLE 2                                                         ______________________________________                                        Operation                                                                              74       72    70     Row Line                                                                             78                                      ______________________________________                                        Read     1        X     X      0      Vdd                                              0        1     0      Vdd    Vdd                                              0        0     1      0      Vdd                                     Program  1        X     X      0      V.sub.GG                                         0        1     0      V.sub.GG                                                                             V.sub.GG                                         0        0     1      0      V.sub.GG                                ______________________________________                                    

It will be understood that the logic function performed by the circuitof FIG. 4 is exemplary, being selected solely for purposes ofillustrating the application of the principles of the present inventionto a specific decoding instance. There is no particular significance tothe particular logic function selected.

As can be seen, because of the requirement that the row line 15 mustcarry V_(GG) in program mode, devices 60, 62, 64, 66 and 68 are all highvoltage devices. Thus, as mentioned above, this prior art decoder isslower than desired.

Referring now to FIG. 5, a new decoder 16 according to the preferredembodiment is shown, implementing the same logic function as thatimplemented in the circuit of FIG. 4. In this circuit only one highvoltage device is needed in the decoding logic function, namelyN-Channel pass-gate device 82.

Also included in this circuit are low voltage p-channel devices 84 and86 and low voltage n-channel devices 88 and 90, all interconnected asshown. Input 92 is provided to receive a voltage, Vph, which is at Vddonly in program mode, and during Read mode is at a "boot" level voltageVbb, which is derived from Vdd, but pumped up to about seven or eightvolts. Input 94 is provided to receive the third logic signal, whileinput 96 is provided to receive the first logic signal. Vdd is appliedto port 98, while port 100 is connected to ground. As before, the outputis row line 15.

The row line 15 passes through the array 1, and continues to a keepercircuit 102 on the opposite side of the array 1. Keeper circuit 102comprises high voltage p-channel devices 104 and 106 and high voltagen-channel device 108, interconnected as shown. V_(GG) is applied to port110, while port 112 is connected to ground.

It will be appreciated that decoder circuit 16 performs the same logicalfunction as the prior art decoder shown in FIG. 4. However, only onehigh voltage device, pass-gate 82 is needed in the logic decode portion,the actual logic function being performed by the other, low voltagedevices 84, 86, 88 and 90. Thus, decoder circuit 16 is significantlyfaster than the circuit shown in FIG. 4, and in addition, the fasterperformance provided by these low voltage devices is scalable as thecircuit is implemented in smaller technologies.

The keeper circuit operates as follows. During program mode, when thedecode circuit 16 decodes a "select" condition for its row line 15, alogic level "1", indicating a select, appears on node N. Pass-gate 82,being enabled, passes this voltage, less Vt, to row line 15. This selectvoltage on line 15 is applied to the gate of device 108 in keepercircuit 102, turning it on which pulls the gate of device 104 to ground,turning device 104 on. The source of device 104 being connected toV_(GG), when it is turned on it pulls the row line 15 up to V_(GG),which is the desired voltage for the program mode. Finally, with rowline 15 being pulled to V_(GG), device 106 is turned off, which preventsV_(GG) from being shorted to ground through device 108 during selectionof row line 15 in program mode. When decode circuit 16 drives node N tozero, indicating deselect of row line 15, row line 15 is pulled to "0"which turns off device 108 and turns on device 106. With device 106 on,the gate of device 104 is pulled high turning device 104 off. Thus, thekeeper circuit pull-up function is terminated, and row line 15 remainsat logic level "0". Note that row select and deselect occur in READmode. When the program mode starts port 110 is switched from Vdd toV_(GG).

The pass gate 82 functions to protect the low power devices duringprogram mode, since its gate is held at Vdd. This prevents the highvoltage generated by the keeper circuit 102 from feeding back throughpass gate 82. Thus the low voltage devices 84, 86, 88 and 90 areprotected.

The pass gate 82 also functions, during Read mode, to beneficiallyeliminate the Vt voltage drop otherwise taken from the decoded row linesignal applied at the input thereof, by having its gate held at Vbbduring Read mode. This important function aids significantly as devicesizes are shrunk and, correspondingly, logic level voltages decrease.

Convertible Array

The Flash memory array 1 may be converted to a ROM array after, e.g.,the program stored in Flash memory array 1 is proven, and it is desiredto begin high volume production. Essentially, the floating gate of thecells 10 (FIG. 1) is eliminated, and the high voltage circuitry, notneeded for the permanent ROM circuit, is not enabled. The novelarrangement, described hereinabove, for the provision of the highvoltage circuitry makes this elimination of the high voltage circuitryrelatively easy, as will now be described. The elimination of thefloating gate is shown schematically in FIGS. 6 and 7, FIG. 6 showing aportion of the array including four cells, C0-C3, and FIG. 7 showing thesame portion of the array with the floating gates removed.

FIG. 8 shows how the high voltage circuitry is disabled. As can be seen,the gate of pass-gate 82 is left unconnected to any circuit wire, and ashort 114 is laid between the source and drain, effectively eliminatingits function in the circuit. In addition, the other end 116 of row line15 is left unconnected to keeper circuit 102. The keeper circuit 102itself is not wire interconnected. All of the foregoing measureseliminating the high voltage circuitry is effected in the chip wiringusing known techniques. Significant, however, is that only the wiremasks need be changed to do this, and those changes to the wire masksare minimal. Thus costly wire re-routing is avoided.

The change in the process steps to eliminate the floating gate will nowbe described. FIG. 9 shows a top view of a section of array 1 in theprocess of being formed on a silicon substrate. This is a similar viewto that in FIG. 2, but an earlier stage in the processing is shown inFIG. 9. As can be seen, channels Ch have been formed, and the firstpolysilicon layer 14 has been formed. The first polysilicon layer 14includes strips 118 that are etched out as described above to define theedges of the floating gates that are formed in subsequent stages. FIG.10 shows the floating gates 14 formed, as described above, during theetch step that creates the floating gates and control gates, with thestrips 118 confining their extent to their individual cells. To form theROM array, which involves the elimination of the floating gate, thesteps of depositing the first polysilicon layer 14 and of forming strips118, are eliminated. The following columns show the steps of the ROMformation process next to the Flash memory formation process:

    ______________________________________                                        FLASH         ROM (with no Power or HV)                                       ______________________________________                                        Start         Start                                                           ALIGN.sub.-- 0                                                                              ALIGN.sub.-- 0                                                  HV.sub.-- NWELL                                                                             .sub.--                                                         HV.sub.-- PWELL                                                                             .sub.--                                                         LV.sub.-- NWELL                                                                             LV.sub.-- NWELL                                                 LV.sub.-- PWELL                                                                             LV.sub.-- PWELL                                                 MOAT          MOAT                                                            C/S           C/S                                                             ARRAY VT      --                                                              SLIT          --                                                              POLY1         --                                                              LVPVT         LVPVT                                                           LVNVT         -- blanket implant                                              --            ROM.sub.-- VT -- high VT for selected bits                      POLY2         POLY2                                                           STAC          --                                                              ARRAY.sub.-- SD                                                                             --                                                              NSD           NSD*                                                            PSD           PSD*                                                            CONT          CONT*                                                           METAL1        METAL1                                                          VIA           VIA                                                             METAL2        METAL2                                                          PO            PO                                                              22 Masks      15 Masks                                                        ______________________________________                                    

The above steps correspond to those described in conjunction with thetext describing FIGS. 3(a)-3(k). By way of clarification, however, notethe following. ALIGN₋₋ 0 is a conventional initial alignment step; HV₋₋NWELL is the step of formation of Deep N-wells 31 for high voltagep-channel devices; HV₋₋ PWELL is the step of formation of P-wells forhigh voltage n-channel devices which, it will be recalled is notdescribed hereinabove, but which is a well known process step; LV₋₋NWELL is the step of formation of N-wells for low voltage p-channeldevices, also not described hereinabove, but also a well known processstep; LV₋₋ PWELL is the step of formation of P-wells 33 for low voltagen-channel devices and the memory array p-well; MOAT is the step ofcreating the isolation regions 41 of oxide; C/S is the step of formationof the channel stops; ARRAY VT is the step of threshold-voltage-adjustimplant in the memory array; POLY1 is the step of deigning the firstpolysilicon layer outside the memory array; SLIT is the step of removingthe strips that define the ends of the floating gates; LVPVT is theimplantation step by which the low voltage p-channel device thresholdvoltage is determined; LVNVT is the implantation step by which the lowvoltage n-channel device threshold voltage is determined; ROM₋₋ VT is apatterned implantation step by which the threshold voltage for selectedbits in the ROM array is determined; POLY2 is the step of defining thesecond polysilicon layer; STAC is the step by which the control andfloating gates are formed by etching; ARRAY₋₋ SD is an implant step bywhich the memory array sources and drains are formed; NSD is the step bywhich the n-channel source drain regions are formed; PSD is the step bywhich the p-channel source drain regions are formed; CONT is the step ofcontact formation; METAL1 is the step of first level wiring; VIA is viaformation for METAL1/METAL2 interconnect; METAL2 is the step of secondlevel wiring; and PO is the step of passivation.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims.

What is claimed is:
 1. A method for converting an integrated circuit including a non-volatile programmable memory containing predetermined programming to a read-only memory (ROM) programmed according to said predetermined programming, comprising the steps of:fabricating a first integrated circuit including a memory having the following elements to test said predetermined programming:an array of memory cells, each cell having a floating gate therein for the storage of charge thereon to program the cell and a control gate for reading the cell, the array having a plurality of row lines, a plurality of column lines and a plurality of output lines, a circuit having a plurality of input lines for each row in the array, and having as outputs the row lines, comprisinga logic circuit associated with each row line, comprising a plurality of low power logic devices interconnected to perform a predetermined logic function on the signals on said input lines to apply a signal to a selected one of said row lines, and a high power pass device associated with each row line, having one of its source and drain connected to the control gates of a row of memory cells in said array, having the other of its source and drain connectable to a row line and having its gate connectable to a second voltage, lower than said first voltage, so as to couple said signal on said row node to said row line, and a keeper circuit associated with each row line and connectable to said high voltage for sensing said signal on said row line and in response thereto for coupling said high voltage to said row line; and fabricating a second integrated circuit, like said first integrated circuit except whereinthe gate of said high power pass device may be left unconnected to other wiring, the source and drain of said high power pass device are shorted together so as to eliminate the function of said high power pass device, the row line is left unconnected to said keeper circuit, and said memory cells are formed without a floating gate. 